Effect of Asphaltenes on the Stability of Water in Crude Oil Emulsions.

Abstract

The presence of asphaltene, especially in heavy crude oil, causes difficulties in the de-watering/desalting process, which is the initial step of crude oil pretreatment. This study investigates the effect of asphaltenes on the stability of crude oil emulsions using a simulated oil system composed of toluene and n-heptane. It was found that asphaltenes behave like conventional surfactants, adsorbing at the oil-water interface and reducing interfacial tension. The critical aggregation concentration (CAC) of W/O emulsions formed from a toluene and n-heptane mixture (7:3 volume ratio) was found to be 0.05 g/L. When the asphaltene concentration was greater than CAC, the asphaltene aggregated into clusters, forming a viscoelastic interface film that enhanced the strength of the emulsion droplets. At an asphaltene concentration of 0.01 g/L, the storage modulus (G') and loss modulus (G″) were 1.12 Pa·s and 8.94 Pa·s, respectively. The storage modulus G' was less than the loss modulus G″, indicating that the viscoelastic nature of the emulsion, and both the G' and G″ of the emulsions increased with the increasing asphaltene concentration. When the concentration reached 11 g/L, G' and G″ were 1033 Pa·s and 416 Pa·s, respectively, with G' exceeding G″, indicating that the emulsion became more stable. Moreover, increasing the solvent aromaticity led to a rise in the CAC of asphaltene, which was favorable for breaking emulsions. The findings imply that reducing the asphaltene content at the interfaces of oil-water by adding an aromatic solvent or blending with light crude oil is a feasible way to break the emulsion and further dewater and desalt.